The disclosure relates generally to the field of separation of particles such as spheres, cells, viruses, and molecules. In particular, the disclosure relates to separation of particles based on their flow behavior in a fluid-filled field of obstacles in which advective transport of particles by a moving fluid overwhelms the effects of diffusive particle transport.
Separation of particles by size or mass is a fundamental analytical and preparative technique in biology, medicine, chemistry, and industry. Conventional methods include gel electrophoresis, field-flow fractionation, sedimentation and size exclusion chromatography. More recently, separation of particles and charged biopolymers has been described using arrays of obstacles through particles pass under the influence of fluid flow or an applied electrical field. Separation of particles by these obstacle-array devices is mediated by interactions among the biopolymers and the obstacles and by the flow behavior of fluid passing between the obstacles.
A variety of microfabricated sieving matrices have been disclosed for separating particles (Chou et. al., 1999, Proc. Natl. Acad. Sci. 96:13762; Han, et al., 2000, Science 288:1026; Huang et al., 2002, Nat. Biotechnol. 20:1048; Turner et al., 2002, Phys. Rev. Lett. 88(12):128103; Huang et al., 2002, Phys. Rev. Lett. 89:178301; U.S. Pat. Nos. 5,427,663; 7,150,812; 6,881,317). These matrices depend on accurate fabrication of small features (e.g., posts in a microfluidic channel). The accuracy with which small features can be fabricated is limited in all microfabrication methods, especially as feature size decreases. The strength and rigidity of materials in which small features of fabricated can also limit the practical usefulness of the fabricated device. Furthermore, the small size of the gaps between obstacles in such matrices can render the matrices susceptible to clogging by particles too large to fit between the obstacles. Micrometer-and nanometer-scale manufacturing also require state-of-the-art fabrication techniques, and devices fabricated using such methods can have high cost.
Previous bump array (also known as “obstacle array”) devices have been described, and their basic operation is explained, for example in U.S. Pat. No. 7,150,812, which is incorporated herein by reference in its entirety. Referring to FIGS. 3 and 4 of U.S. Pat. No. 7,150,812, a bump array operates essentially by segregating particles passing through an array (generally, a periodically-ordered array) of obstacles, with segregation occurring between particles that follow an “array direction” that is offset from the direction of bulk fluid flow or from the direction of an applied field.
At the level of flow between two adjacent obstacles under conditions of relatively low Reynold's number, fluid flow generally occurs in a laminar fashion. Considering the volumetric flow between two obstacles in hypothetical layers (e.g., modeling the flow by considering multiple adjacent stream tubes of equal volumetric flow between the obstacles, as shown in FIG. 8 of U.S. Pat. No. 7,150,812), the likelihood that fluid in a layer will pass on one side or the other of the next (i.e., downstream) obstacle is calculable by standard methods (see, e.g., Inglis et al., 2006, Lab Chip 6:655-658). For an ordered array of obstacles offset from the direction of bulk fluid flow, the arrangement of the obstacles will define an array direction corresponding to the direction in which the majority of fluid layers between two obstacles travels. A minority of fluid layers will travel around the downstream obstacle in a direction other than the array direction.
The path that a particle passing between the two obstacles will take depends the flow of the fluid in the layers occupied by the particle. Conceptually, for a particle having a size equal to one of the hypothetical fluid layers described in the preceding paragraph, the particle will follow the path of the fluid layer in which it occurs, unless it diffuses to a different layer. For particles larger than a single fluid layer, the particle will take the path corresponding to the majority of the fluid layers acting upon it. Particles having a size greater than twice the sum of the thicknesses of the minority of layers that travel around a downstream obstacle in the direction other than the array direction will necessarily be acted upon by more fluid layers moving in the array direction, meaning that such particles will travel in the array direction. This concept is also illustrated in FIGS. 5-11 of U.S. Pat. No. 7,150,812. Thus, there is a “critical size” for particles passing between two obstacles in such an array, such that particles having a size greater to that critical size will travel in the array direction, rather than in the direction of bulk fluid flow and particles having a size less than the critical size will travel in the direction of bulk fluid flow. Particles having a size precisely equal to the critical size have an equal chance of flowing in either of the two directions. By operating such a device at a high Peclet number (i.e., such that advective particle transport by fluid layers greatly outweighs diffusive particle between layers), the effects of diffusion of particles between fluid layers can be ignored.
A method of improving the separating ability of obstacle arrays without requiring a decrease in the size of the array features or the accuracy of microfabrication techniques used to make them would be highly beneficial. The present invention relates to such methods and obstacles arrays made using such methods.